1. Field of the Invention
The present invention relates to a support for an oxygen separation membrane element and the element using the same.
2. Description of Related Art
For example, an oxygen separation membrane element using a mixed conductor solid electrolytic membrane is known. A thinner electrolytic membrane provides higher speed of oxygen transport and, accordingly, higher separability or higher capability of separation. Grounded upon this, a porous support having a plurality of pores which penetrates in the thickness direction, on which a thin electrolytic membrane is formed, is employed for supporting the membrane thereon to be a structure of asymmetric membrane, in order to achieve mechanically sufficient strength, instead of using the sole membrane, as disclosed in JP 2813596 B and JP 2003-210952 A.
It is considered that the oxygen separation membrane element formed as such as a cylindrical shaping, or a pipe shaping, is preferred for a practical use such as a gaseous separation membrane element in a chemical plant, as disclosed in JP 2002-083517 A and JP 2002-292234 A. A cylindrical element is useful and easy to manage in sealing and building a large apparatus with a number of it in comparison with a laminated plate structure. Furthermore, it is advantageous that a small-sized apparatus having a plurality of elements closely located one another and bundled can be provided.
As discussed above, the cylindrical porous support is advantageous for practical use and spread. However, with an asymmetric membrane structure, gaseous diffusion (capability) of the porous support directly affects the capability of the apparatus. Accordingly, a support of high gaseous diffusion capability is required for fabricating an oxygen separation membrane element of high separability. And a support having high mechanical strength and of high affinity with the electrolytic membrane is required for fabricating an oxygen separation membrane element having high durability. Furthermore, easy and low-cost fabrication is expected for practical use.
Various materials, pore forming agents to be added, burning methods and so on have been suggested for fabricating the porous support to support the electrolytic membrane. JP 06-069907 B1, for example, discloses a support of La1-xSrxMnO3 which is synthesized at 1000-1400° C., ground to be 2-10 μm in an average diameter, formed, and burnt at 1300-1600° C. JP 2002-097083 A, for instance, discloses a porous support which is a formed and burnt mixture of mixed conductor oxide material of AFexO3-δ (where A is at least one selected from a group of Ba, Sr and Ca) and resin. It shows a way to change the porosity by controlling the amount of resin to be mixed, the forming pressure and/or the burning temperature. JP 2003-210952 A, for example, discloses another support. A mixture of ABB′O3 (where A is a metallic component coordinated by twelve oxygen atoms, and B and B′ are components each coordinated by six oxygen atoms) and carbon beads is formed, burnt in an oxidization mood to burn out the carbon beads to form pores in which the carbon beads were present. JP 09-132459 A and JP 09-087024 A disclose another supports. A mixture of lanthanum series perovskite material and fine carbon powder having a large specific surface area, for instance, carbon powder of 1-10 μm in the average diameter having not less than 200 m2/g of the specific surface area, is burnt in the oxidization mood to burn out the carbon powder to form the pore. Any one of the above-mentioned methods provides the porous support having a pore diameter and porosity such that the pores do not impede the oxygen transport.
The recent development of the solid electrolytic membrane and catalyst in their quality or capability causes such a situation where the quality or capability of the oxygen separation membrane element depends upon the gaseous diffusion (capability) of the porous support. As a result of it, the further development of the gaseous diffusion (capability) of the porous support is sought.
Table 1 shows the relationship between the theoretical amount of oxygen diffusion, thickness of the membrane and porosity of a porous support fabricated such that the support has not been perfectly sintered (referring to as the “Imperfect Sintering Method”) as shown in JP 06-069907 B1 and JP 2002-097083 A. These theoretical amounts are calculated in Equation 1, Fick's equation, shown below. The permeation (amount) N in Equation 1 corresponds to the theoretical amount of oxygen diffusion in Table 1. A value between 1 and 6 is in general applied to the tortuosity factor τ that is reflective of the porous structure, and, for instance, a value of about 4 (τ=4) is often applied to the conventional porous structure. Dg is a corrected value of the diffusion constant Do in consideration of the temperature and pressure, and the effective diffusion constant Dg′ is a corrected value of the diffusion constant Dg with the porosity α and the tortuosity factor τ. Then m is a constant reflective of the “rigidity” or “hardness” of molecules and a value in a range of 1.5-2.0 is applied to it, for example, a value of 1.75 is applied to m (m=1.75) for an oxygen molecule. The diffusion coefficient Do of oxygen in the air at To and Po is 0.178 (cm2/sec). The distance x corresponds to the thickness of the support.
TABLE 1Theoretical Amount of Oxygen Diffusion in Imperfect Sintering Method (cc/min/cm2)Thickness of Support (mm)Porosity (%)0.20.40.60.81.01.21.41.61.82.02.22.42.6108.94.53.02.21.81.51.31.11.00.90.80.70.72017.88.95.94.53.63.02.52.22.01.81.61.51.43026.713.48.96.75.34.53.83.33.02.72.42.22.14035.617.811.98.97.15.95.14.54.03.63.23.02.75044.522.314.911.18.97.46.45.65.04.54.13.73.46053.426.717.813.410.78.97.66.75.95.34.94.54.17062.331.220.815.612.510.48.97.86.96.25.75.24.88071.235.723.817.814.311.910.28.97.97.16.55.95.59080.140.126.720.116.013.411.510.08.98.07.36.76.2Fick's Equation
                    N        =                                            -                                                Dg                  ′                                RT                                      ⁢                                          ⅆ                p                                            ⅆ                x                                              =                                    -              Dp                        ⁢                                          ⅆ                p                                            ⅆ                x                                                                        Eq        .                                  ⁢        1            
where Dg′=α Dg/τ, Dg=Do (T/To)m Po/P;
N (mol/sec/cm2): Amount of permeation;
Dg (cm2/sec): Diffusion constant at a specified temperature T and under a specified pressure P;
Dp (mol/cm/sec/Pa): Diffusion constant per a specified pressure;
Dg′ (cm2/sec): Effective diffusion constant;
Do (cm2/sec): Diffusion constant at the temperature To (=273 K) and under the pressure Po (=1.01325×105 Pa);
α: Porosity;
τ: Tortuosity factor;
R (J/K/mol): Gas constant (=8.31);
T (K): Temperature;
x (cm): Distance;
P (Pa): Pressure;
m: Constant reflective of the “rigidity” of molecules;
As apparent in Table 1, the smaller the porosity is and the thicker the thickness of the support is, the smaller the theoretical amount of oxygen diffusion becomes. It is experimentally found that the thickness of not less than 2 mm of the support is required for mechanically sufficient strength. Provided that grains in a porous support are in contact with one another, the porosity of the porous support fabricated in the Imperfect Sintering Method reaches 40% in maximum. The mechanical strength of a support remarkably reduces since the bound points between the grains reduce if the percentage exceeds 40%. Consequently, the practical amount of oxygen diffusion can be taken only from an area at the upper right, with the thickness of not less than 2 mm and the porosity of not larger than 40%, in Table 1. Then the theoretical amount of oxygen diffusion is limited in the range of 0.7-3.6 cc/min/cm2 as shown in Table 1. As a result, a small amount of oxygen diffusion of the support has impeded an increase of the oxygen permeation amount of the oxygen separation membrane element, even with improvements in capabilities of the solid electrolytic membrane and others.
There is suggested a porous support fabricated using electrolytic material. It increases the reached oxygen amount to the electrolytic membrane because not only oxygen molecules permeate through pores of the porous support but also oxygen ions that are ionized permeate through the structure of the electrolytic material. Disadvantageously, the support is required to be thicker to achieve the equal mechanical strength to the conventional one because the electrolytic material is more fragile (lower in mechanical strength) in itself than such as alumina that is employed for the conventional one. Consequently, the amount of oxygen diffusion cannot be increased, and then it is difficult to sufficiently increase the amount of oxygen permeation.
JP 3540495 B and JP 11-099324 A disclose an instance of a separation membrane element having the membrane on the support. It is a hydrogen separation membrane element having a support of a metallic base pipe with a plurality of through-holes for ventilation and a metallic layer capable of hydrogen permeation on the outer surface of the support. Such through-holes that linearly extend in the thickness direction of the support provide remarkably smaller ventilation resistance than bent pores which extend through in the thickness direction of the support. Consequently, the capability of gaseous diffusion can be remarkably improved. Table 2 shows the relationship between the theoretical amount of oxygen diffusion, thickness of the membrane and porosity of a porous support having through-holes. It is found in Table 2 that the amount of oxygen permeation can be remarkably increased.
TABLE 2Theoretical Amount of Oxygen Diffusion through Through-holes (cc/min/cm2)Thickness of Support (mm)Porosity (%)0.20.40.60.81.01.21.41.61.82.02.22.42.61035.617.811.98.97.15.95.14.54.03.63.23.02.72071.235.723.817.814.311.910.28.97.97.16.55.95.530106.853.535.726.721.417.815.313.411.910.79.78.98.240142.471.347.535.728.523.820.417.815.814.313.011.911.050178.089.159.444.635.729.725.522.319.817.816.214.913.760213.6107.071.353.542.835.730.626.723.821.419.417.816.570249.2124.883.262.449.941.635.731.227.725.022.720.819.280284.8142.695.171.357.047.540.735.731.728.525.923.821.990320.4160.4107.080.264.253.545.840.135.732.129.226.724.7
The base pipe in JP 3540495 B and JP 11-099324 A is prepared for a metallic porous support for hydrogen separation and cannot serve by itself for oxygen separation. The support for oxygen separation that is formed of ceramic is preferred in order to achieve anti-reduction, anti-moisture and high mechanical strength because it is disclosed at high temperature, under high pressure, in a reduction mood and moisture mood. A metallic cylinder is easy to provide with through-holes on it, however, a ceramic cylinder that is hard to machine, that is, inferior in machinability, is considerably difficult to provide with a plurality of through-holes on it. To provide a molded or sintered support with through-holes by machining as shown in JP 3540495 B and JP 11-099324 A requires much hard labor because only one to a few, not reaching ten holes in maximum, through-holes can be provided. Furthermore, complex control is required to form through-holes on a curved surface, to realize accuracies in such as positioning, dimensions and shaping.
It is therefore an object of the present invention to provide a support for oxygen separation membrane element that has high capability in gaseous diffusion and is easy to fabricate, and oxygen separation membrane element having the membrane with high performance in its oxygen permeation speed.